The invention relates to an electrical focal plane shutter, and more particularly to such a shutter of the type in which the second blind of the focal plane shutter is restrained from running by an electromagnet which is controlled by an electromagnet control device including a light receiving element which is used for photometry.
As is well known, an electrical focal plane shutter is constructed so that the first blind is caused to run upon shutter release, while the running of the second blind is constrained by an electromagnet for producing a desired exposure period. One of the problems involved with the shutter of this kind is the fact that when a power supply for the electromagnet is exhausted or its output substantially reduced, the timing at which the second blind is caused to run becomes no longer precisely controllable, thus preventing a proper exposure. Another difficulty occurs with the electricl focal plane shutter of the type described in a high speed operation such as 1/1000 to 1/2000 second in that if a diaphragm aperture is preset, the length of time during which the shutter should be open is limited to a very small value to thereby result in an almost instantaneous deenergization of the electromagnet, causing a running of the second blind substantially simultaneously with the first blind when it should run in following relationship with respect to the latter, and thus effectively causing a failure of the shutter to open. All the operations of an electrical focal plane shutter take place automatically, and even when an exposure period is determined manually, the movement which occures within the shutter itself depends on electrical means. Where every operation depends on the electrical means, there occurs an inconvenience when the power supply is exhausted or its output reduced in that the shutter is locked, leaving a photographer at a loss as to what to do to correct the malfunction. On the other hand, the provision of a mechanism for operating the shutter mechanically in addition to the electrical means is not advisable in consideration of the space requirement within the camera or the increased cost therefor.